This invention relates to a mechanical stemming construction for explosive loaded blast holes and a method of using such a construction.
Blasting is used in construction and mining to fragment solid rock so that it can be removed. A number of blast holes are drilled and filled with explosive charges which are detonated to produce shock waves that rupture the surrounding rock. There are a number of parameters that govern the effectiveness of a blast, including geologic structure, the size and spacing of the blast holes, the burden (distance to the free face), the type, amount, and placement of explosive, the sequence of detonation, and the stemming technique used.
Stemming is the plugging of the blast hole to prevent the escape of blast gases. This is important because the blast gases perform the primary work of the blast. If the blast gases escape, the effectiveness of the blast is diminished, wasting explosive and requiring additional blasting which entails additional risk and increased drilling, labor, and material costs. Stemming is also important because escaping gases create an overpressure or air concussion causing objectionable noise and possibly causing personal injury or property damage.
Present stemming techniques simply involve filling the blast hole with material. Stemming resists the blast forces because of the inertia of the stemming material, bridging of the stemming material which resists flow, and friction of the stemming material against the blast hole walls. Angular particles are particularly suitable as stemming material because they readily bridge.
With present stemming techniques, the length of the hole required for stemming is sometimes so great that less than an optimum explosive charge may have to be used. This may be the case, for example, in surface blasting in the vicinity of human habitations, or where for geologic or other reasons the blast hole must be kept short. Furthermore, stemming can be expensive and time-consuming, especially where a special stemming material, such as angular rock fragments in a particular size range, is required. Inadequate stemming can lead to "blow outs" where the stemming material is ejected from the hole, which can cause personal injury or property damage.